The present invention relates to a device for connecting an external sound generator, and more particularly to the connecting device having a single jack applicable to the external sound generator which has either one of a 3-pole stereophonic plug and a 2-pole monophonic plug.
Telecommunication systems, such as a transceiver or receiver, usually incorporate a built-in speaker and often additionally incorporate a 2-pole monophonic jack for connecting an external speaker or earphone.
FIG. 1 exemplifies such a telecommunication system incorporating a 2-pole monophonic jack 2. The 2-pole monophonic jack 2, as shown in the figure, comprises a minus contact 4, a plus contact 6, and an armature 8 which opens and closes against the plus contact 6 in response to the insertion and pulling out of a 2-pole monophonic plug 20. The plus contact 6 is connected to a low frequency power amplifier 10 via a DC cut-off capacitor 12. The armature 8 is connected to a built-in speaker 14.
The above-mentioned 2-pole monophonic plug 20 comprises a plus-pole member 22 and a minus-pole member 24, which are electrically connected to an earphone 26, as shown in FIGS. 1 and 2.
Accordingly, inserting the 2-pole monophonic plug 20 into the jack 2 enables the earphone 26 to supply sound. Under noisy circumstances such as a site of construction, it is sometimes required to use an enclosed-type headphone. However, the enclosed-type headphone with the 2-pole monophonic plug is not easily available because of a poor demand. A stereophonic headphone with a 3-pole stereophonic plug is easily available for music appreciation. Therefore, there is a strong demand for such stereophonic headphone instead of the enclosed-type headphone.
Conventionally, using a stereophonic headphone with a 3-pole stereophonic plug has required a 3-pole stereophonic jack. FIGS. 3 and 4 explain such a combination.
FIG. 3 shows a telecommunication system having a typical 3-pole stereophonic jack 42. The jack 42 comprises a minus contact 44, a first plus contact 46, a second plus contact 48, and armatures 50 and 52 which opens and closes against the first and second plus contacts 46 and 48, respectively, by inserting and pulling out a 3-pole stereophonic plug 30. The first and second contacts 46 and 48 are connected to a low frequency power amplifier 10 by way of a DC cut-off capacitor 12. The armature 50 is coupled with a built-in speaker 14.
The 3-pole stereophonic plug 30, meanwhile, comprises a left-channel plus member 32, a right-channel plus member 34, and a minus member 36. As shown in FIG. 4, the left-channel plus member 32 is connected to a left-channel headphone 38, whereas the right-channel plus member 34 to a right-channel headphone 40. The minus member 36 is coupled with both of the left-and right-channel headphones 38 and 40.
In consequence, inserting the 3-pole stereophonic plug 30 into the 3-pole stereophonic jack 42 permits both the left- and right-channel headphones 38 and 40 to provide monophonic sound, respectively.
However, when the 2-pole monophonic plug 20 shown in FIGS. 1 and 2 is inserted into the 3-pole stereophonic jack 42 shown in FIGS. 3 and 4, the minus member 24 of the plug 20 makes contact with both the minus contact 44 and the second plus contact 48, thereby shorting alternatingly the output of the low frequency power amplifier 10, as represented in FIG. 5. (Namely, in FIG. 5, an output voltage Vo=0; a reference Z2 represents an impedance of the monophonic earphone 26.) This short circuit causes an excessive current and leads to a high possibility of damage to the amplifier 10.
This means that an earphone with a 2-pole monophonic plug cannot be used to such a 3-pole stereophonic jack.
To avoid this inconvenience, it is suggested that two dedicated jacks to a 3-pole stereophonic plug and 2-pole monophonic plug be arranged in the same objective apparatus.
However, such arrangement will increase manufacturing cost and require a large space to build in the two types of jack, thus deteriorating a compact system.